Solder and soldering method

ABSTRACT

Soldering paste obtained by mixing solder material with photosetting flux, which hardens by the irradiation of ultraviolet light, is used for reflow soldering. The photosetting flux contains flux base material, flux base material solvent, a photopolymerization initiator and photosetting prepolymer. The soldering paste obtained by mixing this photosetting flux is printed and bonded on the surface of soldered portion of a printed substrate, and is brought into tight contact with the pad portion of a lead of an electronic part to be soldered. The surface of soldering paste exposed is irradiated with ultraviolet light to cause the surface to gel. Thereafter, soldering is effected by means of preliminary heating and solder melting and heating (reflow heating). Since the surface of the soldering paste is thereby hardened by the irradiation of ultraviolet light, no sags occur during preliminary heating, thus making it possible to prevent any defective bridge caused by sags.

This application is a division of application Ser. No. 08/714,308, filedSep. 18, 1996, now U.S. Pat. No. 5,871,592.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to soldering paste used to bond metallicmaterial and a soldering method using the same.

2. Description of Related Art

In order to package a part on the surface of a printed substrate, thereis generally used a reflow soldering method, in which soldering isperformed after the following each process as shown in a process drawingof FIG. 2: soldering paste is bonded to a pat portion for a circuitpattern of the printed substrate by a printing method {circle around(1+L )}, a part is temporarily fixed to the printed substrate by meansof adhesion, etc. {circle around (2+L )}, it is preliminarily heated ata temperature of about 150° C. for several minutes {circle around (3+L)}, and thereafter, is melted and heated at a temperature higher thanthe melt temperature of soldering particles of soldering paste, forexample, 230° C. for scores of seconds {circle around (4+L )}, and iscooled {circle around (5+L )} after the solder is melted for soldering.

The soldering paste consists of a mixed member obtained by dispersingsoldering particles in a flux mixture in which flux, solvent, adhesivemass and the like are mixed, and is printed or bonded to a printedsubstrate or the like by a screen printing method, etc.

The aforesaid printed soldering paste has paste-like viscosity at normaltemperature, has an external shape having the same shape as the screenopening, and maintains a height equal to the thickness of the screen.However, it easily gets fluidized at high temperatures, and mostlystarts being fluidized at a temperature of 50° C. to about 150° C.

When the soldering paste gets fluidized, its soldering particles flowinto contact with the adjacent terminal electrode, causing a so-calleddefective bridge. The occurrence of this bridge causes a significanttrouble in making the circuit pattern pitch narrower (0.5 mm or less)and miniaturizing the printed substrate.

FIG. 3 schematically shows the fluidization of soldering paste, or aprocess in which the defective bridge is caused; FIG. 3A shows a statebefore heating, and FIG. 3B shows the course of the reflow solderingprocess.

In FIG. 3, reference numeral 1 designates a part body such as QFP (QuadFlat Package); 2, a printed substrate; 3, a pad for printing a circuitpattern on soldering paste; 4, lead wire for the part 1; and 5,soldering paste printed on the pad.

Reference numeral 6 designates soldering balls, which are solderingparticles separated and isolated from a mountain of soldering paste 5printed. They adhere to the substrate on printing, or are flowed andappear during heating, and these soldering balls 6 are fluidized in theheating process, thus causing a defective bridge.

In the reflow soldering process, as described above, a printed substrate2, on which soldering paste 5 has been printed, is preliminarily heatedas a preprocess for soldering by melting the solder.

This preliminary heating temperature is about 150° C., lower than atemperature at which solder melts, but the viscosity of flux lowersduring the preliminary heating, the neighborhood of the surface of thesoldering paste 5 printed is fluidized so that the shape at the time ofthe printing is collapsed, thus causing sags 5 a in the soldering paste5.

The sags 5 a flow into contact with the adjacent terminal electrode sidewhen the solder is melted, causing a defective bridge. The defectivebridge due to the sags 5 a occurs more noticeably, the narrower thepitch becomes.

As a soldering method to solve the above-described problem, there isproposed a soldering method for soldering a circuit wiring and lead wireby bringing each joint of the lead wire into tight contact with thecircuit wiring through cream solder, enveloping the each joint and thecream solder in ultraviolet-curing resin, and exposing theultraviolet-curing resin to light to cure it for heating (JapanesePatent Laid-Open No. Hei 7-176856).

In the aforesaid soldering method, however, an extra process forenveloping in the ultraviolet-curing resin is compelled to be added in,for example, the reflow soldering process.

It is an object of the present invention to provide soldering pastecapable of soldering without causing any defective bridge based on sagsin the soldering paste occurring during the aforesaid preliminaryheating, and without adding any extra process, and a soldering methodusing the soldering paste.

SUMMARY OF THE INVENTION

Solder according to the present invention is obtained by mixing soldermaterial with photosetting flux which hardens by the irradiation oflight, and the photosetting flux contains flux base material, flux basematerial solvent, a photopolymerization initiator and photosettingprepolymer.

A soldering method according to the present invention is to, when afirst member and a second member are soldered, use solder obtained bymixing solder material with photosetting flux, which hardens by theirradiation of light, supply the solder onto the surface of the firstmember to bring the second member into tight contact therewith,irradiate the solder surface exposed with light to harden the soldersurface, and heat it for soldering the first member to the secondmember.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process drawing for reflow soldering according to thepresent invention.

FIG. 2 is a process drawing for reflow soldering according to the priorart.

FIG. 3A and 3B are explanatory drawings for occurrence of a defectivebridge.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Soldering paste according to the present invention is characterized by aphotosetting function of soldering flux. Namely, soldering paste capableof preventing the aforesaid sags from occurring by irradiating thesoldering paste printed on a substrate with light to harden the surfaceof the soldering paste.

Although the structure of the aforesaid soldering paste will bedescribed later, when light, for example, ultraviolet light isirradiated, the flux mixture on the surface of soldering paste exposedto outside hardens, but the soldering paste under the lead wire of apart which the ultraviolet light does not reach does not harden, but cannormally solder while it remains paste-like.

When the surface of soldering paste hardens by the irradiation ofultraviolet light, the soldering paste does not sag even in thepreliminary heating process. This enables normal soldering even iffurther heated in the postprocess.

A soldering method according to the present invention using theaforesaid soldering paste is to eliminate the aforesaid defect byprinting, in the reflow soldering process as shown in FIG. 1, onsoldering paste {circle around (1+L )}, temporarily fixing a part{circle around (2+L )}, then irradiating the soldering paste with light{circle around (3+L )} to cause the soldering paste surface to gelbefore preliminarily heating the soldering paste {circle around (4+L )},and preventing the fluidization of the soldering paste surface duringthe preliminary heating {circle around (4+L )}.

By the way, in order not to prevent the soldering particles in the fluxmixture of the soldering paste from combining at the melt temperature ofthe soldering particles, it is necessary to soften them at hightemperature, and to have fluidity, and at least the interior of thesoldering paste must not completely harden.

Therefore, the hardening of the soldering paste by the irradiation oflight {circle around (3+L )} in the soldering method according to thepresent invention is not completely performed, but is preferably lowerthan the gelation level as the pre-stage of hardening.

The flux mixture of the soldering paste is composed of flux basematerial, base material solvent, photosetting prepolymer, aphotopolymerization initiator and the like.

The flux base material is composed of principal ingredients of flux suchas gum rosin and synthetic rosin, organic acid such as adipic acid, fluxactivator such as amine halide, and viscosity, adhesion, cohesion andthixotropic properties modifiers, and combinations widely put topractical use can be used.

As the photosetting prepolymer, simple substances of oligomer such asacrylic ester, methacrylate ester, epoxy acrylate, and urethaneacrylate, their combinations with acrylic monomer and the like aresuitable, and in particular, urethane acrylate, which is free frominhibited cure in oxygen, or its mixed member with acrylic monomer isexcellent because it is cured with ultraviolet light in a short time.

Even when photosetting prepolymer, which is susceptible to inhibitedcure in oxygen, is used, the inhibited cure can be avoided by effectingphotosetting in a nitrogen atmosphere reflow furnace.

As a photopolymerization initiator, benzoin, benzyl, and theirderivatives such as benzoin isopropyl ether are suitable, and furtheramine group may be added as a sensitizing agent.

The base material solvent is used to adjust to viscosity suitable forprinting as the flux mixture, and includes high boiling alcohol, glycolether, ester and the like. It can be selected from among those widelyfor practical use while taking into consideration the compatibility withflux agents and photosetting prepolymer.

After the solder is melted, the ultraviolet-uncured flux mixture whichhas appeared is heated in a high-temperature heating process to follow,and a heat polymerization initiator for heat curing is mixed in advance.

Preferably, the temperature, of the heat polymerization initiator, forstarting to cure the flux mixture is higher than the melt temperature ofsoldering particles, and the mechanism for curing is the same as thatfor the ultraviolet curing. If these curing mechanisms are the same, thecomposition for curing the flux mixture does not become complicated.

As a mechanism for the aforesaid curing, a free-radicalpolymerizationtype is suitable. For the heat polymerization initiator, organicperoxide, which decomposes rapidly at a specified temperature to producefree radical, which cures a flux mixture, can be easily selected. Also,for photopolymerization initiator, the free-radical polymerization typecan be easily selected.

An example of the composition of soldering paste according to thepresent invention is as follows:

Tin solder powder 90.0 parts by weight Flux base material (rosin,  5.5parts by weight activator, viscosity imparting agent) Urethane acrylate(photosetting  2.5 parts by weight prepolymer) Benzoin isopropyl ether(photo-  0.1 part by weight setting initiator) 2-phenoxy ethanol (basematerial  2.0 parts by weight solvent) Peroxy ketal (heat polymerization 0.1 part by weight initiator)

When soldering paste having the aforesaid composition obtained bydispersing soldering particles in a flux mixture to which photosettingproperties are imparted is printed on a substrate, a part is temporarilybonded thereto, and ultraviolet light of 0.1 W/cm² is irradiated ontothe surface of the soldering paste for two minutes, the surface becomesa gelled state though the interior is sufficiently soft.

In short, the irradiation of ultraviolet light causes the surfaceportion exposed of soldering paste to gel, and does not cause a portionhidden under the part terminal to.

Subsequently, when heated in the reflow soldering process (soldermelting and heating process {circle around (5+L )}), the part lead 4 issatisfactorily soldered to the substrate pad portion 3 to form cured,elastic film of the flux mixture on the surface thereof. Soldering balls6 bonded to a substrate 2 when soldering paste is printed are fixed tothe bonded position as they are, and soldering balls 6 due to sags arenot produced, thus preventing any defective bridge due to them.

With a large-sized electronic part having minute pitches, a wickingphenomenon, in which solder on the pad surface is sucked up on the partside, is prone to occur, and with a minute chip part, a Manhattanphenomenon, in which solder on one terminal of the chip melts earlierand surface tension of the solder exerts on the end portion to raise thechip, is prone to occur.

The wicking phenomenon causes defective connection because the amount ofsolder to connect the terminal to the pad is insufficient, and theManhattan phenomenon also causes an irreparable defect.

When elastic film of a flux mixture is formed, the aforesaid wickingphenomenon becomes difficult to occur because the photosetting resinprevents spreading of flux ingredient over the terminal portion or flowof the solder. Of the surface of terminal of the chip part, thephotosetting resin stops the flow of the solder for other than a portionrequiring the flow of solder, and therefore, the occurrence of torque toraise the chip is restrained, thus preventing the Manhattan phenomenon.For this reason, soldering for substrate packaging, in which large-sizedelectronic parts (IC) and minute chip parts are mixedly present, becomesalso easier although it was conventionally difficult.

In the processes of the aforesaid soldering method, by replacing thetemporary part fixing with the light irradiation, the surface ofsoldering paste may be irradiated with ultraviolet light for gelationafter soldering paste is printed, and thereafter the part may betemporarily fixed.

Heretofore, soldering paste prone to cause a defective bridge could notbe practically used even if it has good printing characteristics, butthe soldering paste can be selected for use giving priority to theprinting characteristics according to the soldering method of thepresent invention.

According to solder of the present invention, since no sags of thesolder occur during preliminary heating, it is capable of easily copingwith narrower pitches and miniaturized printed substrates, and bothquality and reliability of products for which soldering has beencompleted are improved.

What is claimed is:
 1. Solder for bonding metallic material, comprising:solder material; and photosetting flux which is mixed with said soldermaterial and hardens by the irradiation of light, wherein saidphotosetting flux is composed of at least flux base material, flux basematerial solvent, a photopolymerization initiator and photosettingprepolymer, and a heat polymerization initiator, which starts thepolymerization of said photosetting flux at a temperature higher thanthe melt temperature of said solder material and which hardens by theirradiation of light.
 2. Solder as defined in claim 1, wherein saidphotosetting prepolymer is urethane acrylate.
 3. Solder as defined inclaim 1, wherein said photopolymerization initiator is benzoin isopropylether.
 4. Solder as defined in claim 1, wherein said photosettingprepolymer is urethane acrylate, and said photopolymerization initiatoris benzoin isopropyl ether.